Technical & Editorial
- Written by Scott Wilkinson
- Published on 15 November 2012
So how is the electric field generated and controlled for each sub-pixel? In the early days of LCD displays, each alignment layer included a series of long, parallel electrodes—horizontal on one plate and vertical on the other plate—with one electrode for each row and column of sub-pixels, as shown in Figure 10. When a voltage was applied to one row and one column, a field was generated at the intersection point.
Unfortunately, this electrode structure, called a passive-matrix design, is not sufficient for a high-quality video display, because the field leaks into the surrounding sub-pixels, causing blurring. Also, the voltage needed to change the orientation of the liquid-crystal molecules is very low, causing the molecules to change their orientation too slowly, which resulted in ghosting.
To solve these problems, a thin-film transistor (TFT) is deposited within each sub-pixel on the first alignment plate (the one closer to the backlight). Also, both sets of addressing electrodes are deposited on the same plate, with the horizontal electrodes insulated from the vertical ones, as shown in Figure 11. This arrangement, called an active-matrix design, limits the electric field to a much smaller area, greatly reducing blurring.
In addition, the second plate (the one farther from the backlight) is coated with a transparent conductive material called indium-tin oxide (ITO). This forms a capacitor with the drain electrodes of the TFTs in the first plate, as shown in Figure 12, increasing the voltage to change the alignment of the molecules, which reduces ghosting.
Of course, in a video image, the field required by each sub-pixel is often different than its neighbors. How can one set of horizontal electrodes and one set of vertical electrodes control each sub-pixel independently? What if two sub-pixels addressed by the same electrode require a different voltage on that electrode?
The answer is surprisingly simple—each frame of the video image is displayed on the screen one row at a time from top to bottom, a process called progressive scanning. This is very similar to progressive raster scanning in an old-style CRT (cathode-ray tube) TV. However, instead of drawing each row from one side of the screen to the other with a beam of electrons as in a CRT, the entire row of sub-pixels in an LCD TV is displayed simultaneously. Then, the next row is activated, and so on until the last row at the bottom of the screen is displayed, after which the entire process is repeated for the next video frame. The horizontal electrode for each row carries a single voltage as it is activated, while the vertical electrodes carry different voltages, providing each sub-pixel in the row with its own, independently controllable electric field.
After each row is displayed, it must remain on the screen with no voltage in the horizontal electrode until the next pass. This requires an additional storage capacitor to be deposited along with each TFT. The capacitor stores just the right amount of electrical energy required for that sub-pixel during that frame, maintaining the liquid-crystal molecules' orientation until the row is activated again for the next frame.
Fig. 13: Sub-pixel structure (Source: fcenter.ru)
In Figure 13, you can see a cross-section of the entire sub-pixel structure, along with a circuit diagram with the TFT, storage capacitor (Cs), and capacitor between the TFT and the ITO layer (Clc).
The process is known as sample-and-hold progressive scanning—the electric field for each sub-pixel in a row is created, or "sampled," and then held until it is updated in the next pass. Interestingly, this is one primary cause of motion blur. (Another, more widely known cause is the response time of the liquid-crystal material.)
Why does sample-and-hold progressive scanning cause motion blur? That will have to wait until the next installment of this article, in which I'll discuss the various limitations of LCD TVs and how they have been overcome—or not.